1. Field of the Invention
The current invention is related to novel designs and methods for manufacturing heating devices that generate heat by oxidation reaction.
2. Relevant Technology
Heating devices for heating human skin are plentiful in the art. The heating element used in a heating device has a significant impact on the design and overall performance of the heating device. As a heat generating medium, the use of elements capable of undergoing an exothermic oxidation reaction has the advantage of being controlled by exposing the oxidation reaction elements to ambient oxygen. For example, an oxidation-based, heat-generating hand warmer may comprise an air-permeable bag containing a heat generating medium. The mixture may comprise loose granules of iron powder, activated carbon, water, salt, and optionally a material such as wood powder for making the medium more porous. The hand warmer is usually stored in an airtight container. After it is taken out of the container, oxygen in ambient air flows into the heat generating medium through the air-permeable bag, and the exothermic oxidation of iron powder in the heat generating medium starts to generate heat.
With apparatus designed for warming the hand or body, the heating devices are not usually manufactured to be compact, and the heating temperature and duration of heat generated are not designed to be precisely controlled. For example, the hand warmer distributed by GRABBER Warmers, Grand Rapids, Mich. 49512 has minimum and maximum temperatures of 40xc2x0 C. and 69xc2x0 C., respectively, and weighs about 20 grams. However, in some situations the size of the heating device and the ability to control temperature and duration of the heat may be important.
U.S. Pat. No. 5,658,583 discloses oxidation-reaction based devices to generate heat for enhancing dermal drug delivery. A heat generating device as disclosed in the patent is a thin, flexible chamber defined by a bottom and surrounding walls made of materials non-permeable to air, and a cover with a structure which allows oxygen in ambient air to flow into the chamber at a proper rate. Inside the thin, flexible chamber is a heat generating medium capable of generating heat when exposed to oxygen. A typical composition of the heat generating medium include activated carbon, iron powder, sodium chloride, fine wood powder, and water in a proper ratio.
In many medical related applications, such as enhancing transdermal drug delivery and regulating injected or implanted controlled drug release systems, the heating device must meet certain criteria for the device to be functional and practical. For example, the device usually needs to be thin and compact. The duration and temperature of the heat generated need to be precisely controlled and reproducible, so that the risk of drug overdose or under dose can be minimized. Additionally it is often desirable to be able to place as much heat generating medium into the chamber as possible, so that the heating device, while compact, can generate heat for sufficient duration. Moreover, the device may need to be sterile and disposable. The design and manufacturing process of the heat generating medium affects the potential applications of the heating device.
The task of designing and efficiently manufacturing heating elements in a heating device can be difficult. For example, one way to place the heat generating medium into a device is to pre-mix all of the ingredients of the heat generating medium together, and then load the pre-mixed heat generating medium into the chamber or a bag. Manufactured in this way, the heat generating medium is in the form of loose granules or coarse powder. The chamber or the bag is then closed and sealed into an air-tight container. However, the difficulties associated with directly loading pre-mixed heat generating medium into a chamber or bag make such an approach undesirable in some situations.
One drawback of the method described above is that, the pre-mixed granules or coarse powder are usually loose so that the quantity of the pre-mixed heat generating medium that can be loaded into a unit volume is not maximized. Additionally, the particles in the pre-mixed granules are often attracted by electric static and fly outside the chamber during loading. Furthermore, the pre-mixed heat generating medium starts to generate heat immediately upon contacting oxygen and starts losing its ability to generate heat later on. To prevent the exothermic reaction from occurring prematurely in such a process, the components need to be mixed, transported, and loaded into the chamber in an oxygen free environment. An oxygen-free environment can be expensive to create and maintain.
These limitations in design and manufacturing can pose serious problems for certain applications, such as in many medically related applications, and when the volume of the chamber is designed to be small. Thus, it would be advantageous to develop a better method for manufacturing oxidation-based heating devices.
In view of the foregoing, it is an object of at least one embodiment of the present invention to increase the amount of heat generating medium per unit volume in a heat generating device.
It is also an object of at least one embodiment of the present invention to provide a more homogenous uniform distribution of heat generating medium in the heating device.
It is another object of at least one embodiment of the present invention to inhibit oxidation of the heat generating medium during the manufacturing process.
It is an additional object of at least one embodiment of the present invention to allow the manufacturing process to be done in normal ambient air.
It is further an object of at least one embodiment of the present invention to ensure that the final water percentage (water ratio) in the heat generating medium of the heating device is as close to the ideal water percentage for the oxidation reaction as is possible.
It is yet another object of the present invention to provide a manufacturing process for making controlled heating devices that use oxidation reactions to generate heat which are suitable for use in medical and therapeutic-related uses.
The current invention is related to novel designs and methods for manufacturing heating devices that generate heat by oxidation reaction.
The limitations discussed have not been successfully addressed in the art until now and thus, prior art devices do not operate with precise heating, duration and temperature and/or are not compact.
Manufacturing approaches addressed in the present invention relate to, first, the difficulty in manufacturing the heat generating medium which is activated by oxygen present in the atmosphere, and second, the difficulty in loading the generating medium into the heating device in such a way as to maximize the amount of medium per unit volume in the device and provide for uniform heating. To reduce the loss of heat generating potential due to untimely reaction of the heat generating medium with ambient oxygen during the manufacturing process, the present invention provides means for inhibiting oxidation in a heat generating medium during the manufacturing process. At an appropriate time during the manufacturing process, the oxidation capacity of the heat generating medium is substantially restored. To address the difficulties involved with loading the heat generating medium into the heating device, the present invention provides a loading facilitator to increase the amount of heat generating medium per volume dispersed within the heating device and to provide an even and uniform distribution of the heat generating medium. When appropriate, the loading facilitator is removed from the heat generating medium within the device. The description below explains in greater detail the aspects of the present invention.
In a typical oxidation-reaction-based heat generating medium, the ratio of water in the medium is critical to the heat generating function. For example, when using a mixture of iron powder, activated carbon, sodium chloride, wood powder and water, if the water ratio is too high relative to the other components, the oxidation reaction either will not take place or will be at such low rates that proper heat cannot be generated. Similarly, if the ratio of water is too low, the reaction will not take place or will not generate the necessary heat. The present invention utilizes the water ratio requirements of oxidation reactions in a novel way that makes the manufacturing of oxidation-based heat generating devices easier, and allows manufacturing to take place in ambient air without losing significant heat generating capacity of the products. Using this method, heating devices which have more precisely controlled heating temperature and duration can be manufactured. In addition, this method also allows precise loading of small quantities of heat generating medium, (e.g., less than 1.0 grams) into very thin and shallow heating device chambers.
In this application, the terms xe2x80x9coxidation-facilitatorxe2x80x9d and xe2x80x9coxidation facilitating substancexe2x80x9d are defined as a substance, the existence of which is necessary for the heat generating medium to generate heat properly upon contact with oxygen. The terms xe2x80x9coxidation-inhibitorxe2x80x9d and xe2x80x9coxidation inhibiting substance are defined as a substances which, if added into an oxidation based heat generating medium significantly inhibits the heat generating capacity of the heat generating medium. Some embodiments of the present invention will employ an oxidation inhibitor rather than an oxidation facilitator. The inhibitor or facilitator can be a single substance, a mixture of substances, or a solution. In one embodiment the facilitator and inhibitor exist in a liquid state at manufacturing and/or application temperature and are therefore referred to as an xe2x80x9coxidation facilitating liquidxe2x80x9d and an xe2x80x9coxidation inhibiting liquid,xe2x80x9d respectfully.